In my opinion, any UML tool is good enough. None of them involve rocket science for basic uml diagramming. They are pretty intuitive.
The tools differ in their features like code generation, different language support etc etc.
Here is good list of various UML tools(both free and licensed) -
Just quick notes. AgroUML and StarUML are good choices among others. But be careful, as long as I knew, AgroUML didn't have undo functionality. StarUML does not support UNICODE in the notes and names. Again as far as my experience goes.
Among the free tools, Jdeveloper 10g and 11g from Oracle, provide very good UML tools with none of the above problems and they are free. They can also be used for code generation and have several other features.
Hope it helped.